Catheterization is used in diagnostic and therapeutic procedures. For example, a cardiac catheter is used for mapping and ablation in the heart to treat a variety of cardiac ailments, including cardiac arrhythmias, such as atrial flutter and atrial fibrillation which persist as common and dangerous medical ailments, especially in the aging population. Diagnosis and treatment of cardiac arrhythmias include mapping the electrical properties of heart tissue, especially the endocardium and the heart volume, and selectively ablating cardiac tissue by application of energy. Such ablation can cease or modify the propagation of unwanted electrical signals from one portion of the heart to another. The ablation process destroys the unwanted electrical pathways by formation of non-conducting lesions. Various energy delivery modalities have been disclosed for forming lesions, and include use of microwave, laser and more commonly, radiofrequency energies to create conduction blocks along the cardiac tissue wall. In a two-step procedure—mapping followed by ablation—electrical activity at points within the heart is typically sensed and measured by advancing a catheter containing one or more electrical sensors (or electrodes) into the heart, and acquiring data at a multiplicity of points. These data are then utilized to select the endocardial target areas at which ablation is to be performed.
The term “radiofrequency” (RF) is commonly used to refer to an alternating current that flows through a conductor. In the case of ablation, RF current flows through biological tissue that contains free ions. The extra cellular fluid present in the tissue provides the electrical conductivity. The tissue conductivity can be represented by tissue impedance. In general, low impedance represents high conductivity and high impedance represents low conductivity.
The application of RF current biological tissue causes heating of tissue. The higher the RF current density in the biological tissue (current per unit area), the higher the resulting temperature. The tissue stops reacting to electrical stimulation when heated above a threshold over a short period.
Another catheter-based ablation procedure is renal denervation (RDN). It is a minimally invasive, endovascular catheter based procedure using radiofrequency ablation aimed at treating medical conditions and diseases, including, for example, hypertension. The sympathetic system fuels the release of certain hormones that affect and control blood pressure. In hypertension, the continued release of low-dose amounts of these hormones can increase blood pressure. Hypertension can be controlled by diet, exercise and drugs. However, resistant hypertension (commonly defined as blood pressure that remains above goal in spite of concurrent use of three antihypertensive agents of different classes) requires more aggressive treatments, including surgery. Resistant hypertension is a common clinical problem faced by both primary care clinicians and specialists. As older age and obesity are two of the strongest risk factors for uncontrolled hypertension, the incidence of resistant hypertension will likely increase as the population becomes more elderly and heavier.
It has been established that severing the renal nerves improves blood pressure. However, this procedure involves surgery and all its attendant risks, and often resulted in global sympathetic denervation below the chest. Being able to de-nervate, or silence, only the renal nerves through a catheter-based system is a crucial development. A small catheter is placed in the femoral artery and access to the nerves is gained through the renal artery. The nerves are woven and embedded in the casings or layers around the renal arteries. By passing an energy source into the renal artery and transmitting a low-dose energy, radiofrequency ablation, through the catheter, inbound and exiting renal sympathetic nerves are exposed to RF current densities. The extent of heating is proportional to the RF power (current density) output. At low current densities, the tissue is heated slowly and contracts because of fluid loss. With the nerves impaired or “denerved” at selected locations along their lengths, sympathetic afferent and efferent activity is interrupted or reduced with beneficial effects, such as a reduction in blood pressure.
Current ablation systems provide electrophysiologist with temperature, impedance and power feedback during an ablation procedure. However, unlike cardiac ablation, such feedback in renal ablation denervation does not readily provide information on acute end point indicating successful ablation. That is, such feedback information does not readily help determine whether renal nerves have been impacted by the ablation. However, renal arteries can be prone to exhibit physiological response during ablation. One response includes the potential for arterial spasming.
During spasming, an artery can suddenly narrow, constricting blood flow through the artery. With a reduced inner diameter, the artery can close in on the ablating electrode, increasing the surface area of the artery in contact with the electrode and hence improving ablation efficiency by increasing the amount of ablation power delivered to the tissue. However, with the increasing amount of ablation power, there is a greater risk for artery stenosis. Renal artery stenosis is undesirable, if not dangerous, because narrowing of the renal arteries prevents normal amounts of oxygen-rich blood from reaching the kidneys which need adequate blood flow to help filter waste products and remove excess fluids. Reduced blood flow may increase blood pressure and injure kidney tissue.
Accordingly, there is a desire for a system and a method of renal arterial ablation which help monitor the potential for renal arterial spasming as an indicator of ablation while controlling the amount of ablation power applied to reduce the risk of undesirable damage to the renal artery as a result of excessive ablation.